1. Field of the Invention
The present invention relates to a semiconductor switch, a transceiver, a transmitter, and a receiver which switch an RF signal between lines.
2. Background Art
There has been a demand to reduce the transmission loss in semiconductor switches which generally operate in millimeter wave bands. In some semiconductor switches, switching devices are shunt-connected to signal transmission lines (that is, they are connected at one end to the signal transmission lines and at the other end to ground) to reduce the transmission loss. FIG. 30 shows a conventional semiconductor switch that includes such switching devices. Specifically, the semiconductor switch, 100, shown in FIG. 30 is a general millimeter wave SPDT (single pole double throw) switch. In the semiconductor switch 100 shown in FIG. 30, each of two branch lines includes two switching devices and two transmission lines in order to enhance the isolation of the branch line when it is in the OFF state. Further, the switching devices are field effect transistors (FET).
In the semiconductor switch 100 shown in FIG. 30, a main terminal Tm for an RF signal is connected to a branch point P by a main transmission line Lmb1 which forms a main line Lmb. The branch point P is connected to a branch terminal T1 through a transmission line Lb11 and a transmission line Lb12 which form a branch line Lb1. The branch point P is also connected to a branch terminal T2 through a transmission line Lb21 and a transmission line Lb22 which form a branch line Lb2. The main transmission line Lmb1 and the transmission lines Lb11, Lb12, Lb21, and Lb22 have the same characteristic impedance. The transmission lines Lb11, Lb12, Lb21, and Lb22 have a length equal to one-quarter of the wavelength, λ, of the RF signal transmitted on these transmission lines.
A switching device SW11 is shunt-connected to the junction between the transmission lines Lb11 and Lb12. That is, one end of the switching device SW11 is connected between the transmission lines Lb11 and Lb12, and the other end is grounded. A switching device SW12 is shunt-connected to the junction between the branch terminal T1 and the transmission line Lb12. Further, a switching device SW21 is shunt-connected to the junction between the transmission lines Lb21 and Lb22.
A switching device SW22 is shunt-connected to the junction between the transmission line Lb22 and the branch terminal T2. A control voltage is applied to the control terminals V1 of the switching devices SW11 and SW12, and another control voltage is applied to the control terminals V2 of the switching devices SW21 and SW22. Since, as described above, the transmission lines Lb11, Lb12, Lb21, and Lb22 have a length equal to one quarter wavelength λ/4 of the RF signal, the impedances of these transmission lines can be controlled by controlling the control voltages applied to the control terminals V1 and V2. That is, the RF signal from the main terminal Tm can be selectively transmitted through one of the two branch lines. This construction allows for reduction of the transmission loss in the semiconductor switch 100.
FIG. 31 shows a semiconductor switch which differs from that shown in FIG. 30 in that the FETs are replaced by diodes. In this case, a control voltage is applied to a diode D11 and a diode D12 through the branch terminal T1, and another control voltage is applied to a diode D21 and a diode D22 through the branch terminal T2 to switch the RF signal between the branch lines. Although in the semiconductor switch 100 shown in FIG. 30 the branch line Lb1 between the branch point P and the branch terminal T1 includes two transmission lines (namely the transmission lines Lb11 and Lb12), in other constructions the branch line Lb1 may include only one switching device and one transmission line. For example, the transmission line Lb12, the switching device SW12, the transmission line Lb22, and the switching device SW22 may be omitted from the construction shown in FIG. 30 to reduce the size of the semiconductor switch 100 at some sacrifice of the isolation of the branch lines.
FIG. 32 is an equivalent circuit diagram of the semiconductor switch shown in FIG. 30 when the switching devices SW11 and SW12 are turned on (by applying a voltage of 0 V to the control terminals V1) and the switching devices SW21 and SW22 are turned off (by applying a voltage less than the pinch-off voltage to the control terminals V2). In this case, each of the switching devices SW11 and SW12 is equivalent to a series connection of an on-resistance (Ron) and a parasitic inductive component (Lp). Therefore, the transmission lines Lb11 and Lb12, which have a length equal to one quarter wavelength λ/4 of the RF signal, act approximately as shorted stubs and hence have high impedance. As a result, the RF signal does not propagate through the transmission lines Lb11 and Lb12.
On the other hand, each of the switching devices SW21 and SW22 is equivalent to a series connection of an off-capacitance (Coff) and a parasitic inductive component (Lp), since a voltage less than the pinch-off voltage is applied to the control terminals V2, as described above. Therefore, the impedances of the transmission lines Lb21 and Lb22 are substantially equal to their characteristic impedances. As a result, the RF signal input from the main terminal Tm propagates through the branch line Lb2 to the branch terminal T2.
FIG. 33 is a Smith chart showing impedances in the equivalent circuit shown in FIG. 32. In this chart, point A indicates the impedance looking toward the main terminal Tm from the branch point P (i.e., looking in the direction of arrow A in FIG. 32), which impedance is hereinafter referred to as the “impedance A”; point B indicates the impedance looking toward the branch terminal T1 from the branch point P (i.e., looking in the direction of arrow B in FIG. 32), which impedance is hereinafter referred to as the “impedance B”; and point C indicates the impedance looking toward the branch terminal T2 from the branch point P (i.e., looking in the direction of arrow C in FIG. 32), which impedance is hereinafter referred to as the “impedance C.” The impedance A of the main line side is equal to the characteristic impedance Zo of the main line and therefore is located at the center of the Smith chart. The impedance C is the combined impedance of the transmission lines Lb21 and Lb22 (which have a length equal to one quarter wavelength λ/4 of the RF signal) and the off-capacitances of the switching devices SW21 and SW22 and is located substantially at the center of the Smith chart. On the other hand, the impedance B is high and is located near the rightmost edge of the horizontal axis of the Smith chart, since the transmission lines Lb11 and Lb12 act substantially as λ/4 shorted stubs and have high impedance as a result of the impedances of the switching devices SW11 and SW12 being very low (equal to the on-resistance).
Referring to the equivalent circuit diagram shown in FIG. 32, the RF signal applied to the main terminal Tm propagates through the main transmission line Lmb1 to the branch point P. However, the RF signal does not go through the transmission lines Lb11 and Lb12, since the impedance B (looking from the branch point P) is high. On the other hand, the impedance C of the branch line Lb2 side that includes the transmission lines Lb21 and Lb22 is approximately equal to the characteristic impedance Zo (i.e., approximately equal to the impedance A). Therefore, the RF signal propagates from the branch point P through the transmission lines Lb21 and Lb22, since the impedance C is conjugately matched to the impedance A. That is, in this case, the branch line Lb1 side (including the transmission lines Lb11 and Lb12) functions as the OFF side, or the RF signal-blocking side, of the switch. On the other hand, the branch line Lb2 side (including the transmission lines Lb21 and Lb22) functions as the ON side, or the RF signal-transmitting side, of the switch. This is accomplished since the voltages described above are applied to the control terminals of the switching devices SW11, SW12, SW21, and SW22.
It should be noted that if the RF signal is applied to the branch terminal of the branch line side which is functioning as the OFF side of the switch, instead of being applied to the main terminal Tm, the RF signal does not reach the main terminal Tm and the other branch terminal since the branch line side is in a high impedance state. On the other hand, if the RF signal is applied to the branch terminal of the branch line side which is functioning as the ON side, the RF signal reaches the main terminal Tm but does not reach the other branch terminal.
It should be noted that semiconductor switches are disclosed in Japanese Laid-Open Patent Publication Nos. 2000-196495, H10-242826, H07-235802, 2002-171186, and 2000-183776. The above Japanese Laid-Open Patent Publication No. 2000-196495 discloses a semiconductor switch having a reduced number of transmission parts, resulting in reduced transmission loss.
The construction of the above conventional semiconductor switch 100 requires that the transmission lines Lb11, Lb12, Lb21, and Lb22 have a length equal to one-quarter of the wavelength of the RF signal. This requirement results in increase of the dimensions of the semiconductor switch, thus preventing reduction of the size of the switch. The construction shown in FIG. 31 in which diodes are used as switching devices also has the same problem. This means that it is not possible to reduce the dimensions of transceivers, transmitters, and receivers using the above semiconductor switch.